1. Field of the Invention
The present invention relates to a method for growing a single crystal, and more particularly to a method for growing a single crystal to be used, for example, for optical parts such as an optical isolator and optical circulator, and high frequency parts such as a magnetostatic wave element and oscillator.
2. Description of the Related Art
Materials for an optical isolator, an important device in optical communication equipment, are required to have small light absorption at the wave band to be used, a large Faraday effect and small temperature fluctuation. Single crystals of magnetic garnet containing iron, typically yttrium iron garnet (Y.sub.3 Fe.sub.5 O.sub.12 ; abbreviated as YIG hereinafter) single crystal, have been developed as materials satisfying the foregoing conditions. YIG is a incongruent melting compound, and a single crystal of the YIG cannot be grown directly from a congruent melting composition. Accordingly, the crystal has been usually grown from a liquid prepared by using a flux having a special composition.
A typical example for producing YIG will be described hereinafter. As hitherto described, YIG is known as an incongruent melting compound, which is dissociated into an orthoferrite (YFeO.sub.3) and a liquid phase when the YIG is heated to 1585.degree. C. YIG can not be obtained by merely solidifying a molten liquid having a stoichiometric composition of the YIG, but the orthoferrite (YFeO.sub.3) precipitates as an initial phase. For producing YIG by growing the YIG single crystal, therefore, iron (III) oxide (Fe.sub.2 O.sub.3) and yttrium (III) oxide (Y.sub.2 O.sub.3) is dissolved as solutes into a solvent prepared by mixing lead oxide (PbO) and diboron trioxide (B.sub.2 O.sub.3) in a crucible made of Pt. A bulk crystal is obtained by slowly cooling this solution after adding a seed crystal by the flux method while in the LPE (Liquid Phase Epitaxy) method, the crystal is deposited on a substrate of a gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12 ; abbreviated as GGG hereinafter) single crystal.
The inventors of the present invention have grown a fiber-like YIG single crystal by a floating zone method (abbreviated as a FZ method hereinafter) using an image furnace heated by YAG laser focusing equipped with a YAG laser on the optical heating apparatus (Japanese Unexamined Patent Publication No. 9-320848). Contamination by impurities from the crucible material can be avoided in the FZ method since no crucible is used, enabling one to select a desirable atmosphere for producing a single crystal with a high melting point. It is also possible to grow a bulk single crystal of the melt-degradation type compound by using a TSFZ (Travelling Solvent Floating Zone) method. Furthermore, the inventors of the present invention have developed a SSFZ (Self-adjusting Solvent Floating Zone) method of growing the fiber-like YIG single crystal using an image furnace heated by YAG laser focusing, intentionally avoiding use of a flux, as disclosed in U.S. patent application Ser. No. 09/035,472 and European patent application No. 98104301.1.
Since YIG itself has a small Faraday rotatory power and has a drawback in that the temperature dependency of the Faraday rotatory power is large, it has been proposed that the Y site be substituted with Bi for solving the former problem while substituting the Y site with a rare earth element such as Tb for solving and latter problem. The YIG single crystal substituted with Bi (abbreviated as Bi:YIG hereinafter) has been commercially available as an optical isolator material produced using a LEP method having a good mass-productivity. However, when one attempts to grow Bi:YIG single crystal by the FZ method, Bi evaporates during preparation of materials due to the high vapor pressure of Bi, and fails to be incorporated into the YIG.
Meanwhile, it was recently reported that a large Faraday rotatory power can be obtained in the single crystal of magnetic garnet in which the Y site in the YIG is substituted with Ce (abbreviated as Ce:YIG hereinafter) using a sputtering method (G. Gomi et. al., J. Appl. Phys, 70 (1991), p7065).
Conventionally, YIG single crystals substituted in high concentration with cerium (Ce) have been prepared by the sputtering method only. However, the single crystal is not satisfactory as a material for use in optical communication equipment since a film of only about 1 to 2 .mu.m is produced by the sputtering method. Although the Ce:YIG single crystal can be also grown by the LEP method, on the other hand, there remains a problem that the incorporated amount of Ce into YIG is small. The TSFZ method has the same tendency as described above. Accordingly, it was impossible to produce a single crystal large enough for use in materials for an optical device with respect to the magnetic garnet single crystal substituted with Ce.